Waiting for 9.4 – Provide moving-aggregate support for a bunch of aggregates.

Provide moving-aggregate support for a bunch of numerical aggregates.
First installment of the promised moving-aggregate support in built-in
aggregates: count(), sum(), avg(), stddev() and variance() for
assorted datatypes, though not for float4/float8.
In passing, remove a 2001-vintage kluge in interval_accum(): interval
array elements have been properly aligned since around 2003, but
nobody remembered to take out this workaround. Also, fix a thinko
in the opr_sanity tests for moving-aggregate catalog entries.
David Rowley and Florian Pflug, reviewed by Dean Rasheed

5 calls to custom_concat + 3 to ms_custom_concat. That's nice. Generally – the larger the “frame" the better these new aggregates will be. Of course there is non-zero cost of calling inversion function, but, if written correctly – it shouldn't be a big problem.

One additional thing to note – moving aggregate can use different “storage type" (that's why I had both stype and mstype). So you can, for example, choose different algorithm and temporary storage for moving aggregates. And that's definitely cool. Thanks guys.